To crush someone's enthusiasm for something by pointing out that no matter how excited they are, what they are doing is both pointless and stupid.

I know this is a bit of a sydoperous moment but you do realise you're just taking photos of a plastic hippo by some kitchen pans right?

I hate to be sydoperous but that frozen african dildo is just going to wake her up the moment you use it and leave you with shit all over your hands

It was with ironic sydopery that Samuel Johnson responded to his contemporaries' claims that he'd accidently left sydoperous out of his dictionary by merely saying "I haven't included it because it's a stupid word that you've clearly just made up isn't it, you fuckingidiot".